Metallic or metallic appearing pigments in organic vehicles have been used for many years in organic coatings serving various aesthetic and functional purposes. Relatively recently, metallic pigments have been used to provide electroconductive coatings for non-metallic electronic housings which coatings have radio frequency interference and electromagnetic interference shielding characteristics. The conductivity of the coatings is evaluated in tests for surface resistance. Surface resistance (or more technically, the resistivity) of a conductive paint is measured in ohms per square. Ohms per square is a non-dimensional measurement which is derived from ohms law and specific resistance equations, and as such is independent of the area of square-faced probes, provided the distance between the probes is equal to the size of the probes. The required level for surface resistance is 0.5 ohm per square or less. The conductive pigment is mixed in a carrier paint system, applied as a uniform layer to a glass microscope slide or a structural foam plastic substrate and dried. Surface resistance is determined by placing a set of flat square-faced probes, having equal surface areas, in contact with the surface of the dried conductive coating and recording the resistance with an ohmmeter. The distance between the probes is such that the surface area between the probes is equal to the area of each probe (e.g., probes 1" square would have exactly 1" between the edges of the probes). The surface resistance is independant of the area of the conductor, as long as the length and width are equal. The test is non-standard, but is comparative.
Provided that electroconductivity characteristics are satisfactory, the electromagnetic shielding characteristics of the coating material are essentially determined by the electromagnetic characteristics of the pigment. For electromagnetic shielding purposes, it is advantageous that the metallic pigment as a whole be a ferromagnetic material of reasonable magnetic saturation and low remanence. As a further point, metallic pigments suitable for electrical and electromagnetic shielding purposes should be as inexpensive as is practical.
It is the object of this invention to provide an inexpensive pigment material or system adapted to be employed in an organic vehicle, the coating material thus provided having, in the dried thin film state, good electrical conductivity and good electromagnetic shielding characteristics.